How to Change Fresh Water Into Brackish Water

Fresh water from a domestic water supply is suitable for tropical and coldwater fish with the addition of a water dechlorinator to make it fish-safe. You can add marine salt to fresh water to make it brackish for fish such as spotted puffers, monos and scats. Some naturally freshwater species prefer elevated salt levels, so you can also house mollies and archer fish in a brackish tank. Brackish water salinity is halfway between saltwater and freshwater, but it is not sufficiently salty to sustain true marine fish.

Things You'll Need

  • Bucket
  • Water dechlorinator
  • Marine aquarium salt
  • Measuring jug
  • Tablespoon
  • Aquarium hydrometer
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Instructions

    • 1

      Fill a bucket with tap water using a measuring jug, making a note of how many gallons of water you pour into the bucket. Add a water dechlorinator to remove chlorine and chloramine, according to the manufacturer's dosage guidelines and mix the water thoroughly.

    • 2

      Stir in 1 tbsp. of synthetic marine salt per gallon of fresh water in the bucket. Marine salt contains essential minerals required by salt-loving fish and although tropical aquarium salt does the same job, it lacks natural sea minerals which can lead to fluctuations in pH and an unstable environment. Agitate the water, making sure no salt is left undissolved.

    • 3

      Place an aquarium hydrometer in the bucket to measure the specific gravity, or salinity of your water. Specific gravity represents a liquid's density in relation to the density of fresh water as a unit-less measure. Fresh water is indicated as 1.000 and sea water is 1.023, so aim for a reading of 1.012 for average brackish salinity.

    • 4

      Add additional salt 1 tbsp. at a time, stirring the water continuously, until the hydrometer gives a stable reading of at least 1.012. The salinity of brackish water varies in nature, so you can safely add up to 4 tbsp. of salt per gallon of fresh water, depending on the type of fish you want to keep in your brackish tank.

    • 5

      Ensure all salt crystals are completely dissolved and pour the brackish water into your aquarium. Repeat the preparation process for each bucket of water until your aquarium reaches the required level.

    • 6

      Establish a stable water temperature of 70 to 85 degrees Fahrenheit depending on the species of brackish fish in your tank and maintain a pH level between 7.8 and 8.3. Crushed coral or aragonite in the substrate keeps the water pH high and alkalinity at a suitable level.